Macrophage-specific de Novo Synthesis of Ceramide Is Dispensable for Inflammasome-driven Inflammation and Insulin Resistance in Obesity.

Dietary lipid overload and calorie excess during obesity is a low grade chronic inflammatory state with diminished ability to appropriately metabolize glucose or lipids. Macrophages are critical in maintaining adipose tissue homeostasis, in part by regulating lipid metabolism, energy homeostasis, and tissue remodeling. During high fat diet-induced obesity, macrophages are activated by lipid derived "danger signals" such as ceramides and palmitate and promote the adipose tissue inflammation in an Nlrp3 inflammasome-dependent manner. Given that the metabolic fate of fatty acids in macrophages is not entirely elucidated, we have hypothesized that de novo synthesis of ceramide, through the rate-limiting enzyme serine palmitoyltransferase long chain (Sptlc)-2, is required for saturated fatty acid-driven Nlrp3 inflammasome activation in macrophages. Here we report that mitochondrial targeted overexpression of catalase, which is established to mitigate oxidative stress, controls ceramide-induced Nlrp3 inflammasome activation but does not affect the ATP-mediated caspase-1 cleavage. Surprisingly, myeloid cell-specific deletion of Sptlc2 is not required for palmitate-driven Nlrp3 inflammasome activation. Furthermore, the ablation of Sptlc2 in macrophages did not impact macrophage polarization or obesity-induced adipose tissue leukocytosis. Consistent with these data, investigation of insulin resistance using hyperinsulinemic-euglycemic clamps revealed no significant differences in obese mice lacking ceramide de novo synthesis machinery in macrophages. These data suggest that alternate metabolic pathways control fatty acid-derived ceramide synthesis in macrophage and the Nlrp3 inflammasome activation in obesity.

Mechanism of protein biosynthesis in mammalian mitochondria.

Protein synthesis in mammalian mitochondria produces 13 proteins that are essential subunits of the oxidative phosphorylation complexes. This review provides a detailed outline of each phase of mitochondrial translation including initiation, elongation, termination, and ribosome recycling. The roles of essential proteins involved in each phase are described. All of the products of mitochondrial protein synthesis in mammals are inserted into the inner membrane. Several proteins that may help bind ribosomes to the membrane during translation are described, although much remains to be learned about this process. Mutations in mitochondrial or nuclear genes encoding components of the translation system often lead to severe deficiencies in oxidative phosphorylation, and a summary of these mutations is provided. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

Mitochondrial genome instability and ROS enhance intestinal tumorigenesis in APC(Min/+) mice.

Alterations in mitochondrial oxidative phosphorylation have long been documented in tumors. Other types of mitochondrial dysfunction, including altered reactive oxygen species (ROS) production and apoptosis, also can contribute to tumorigenesis and cancer phenotypes. Furthermore, mutation and altered amounts of mitochondrial DNA (mtDNA) have been observed in cancer cells. However, how mtDNA instability per se contributes to cancer remains largely undetermined. Mitochondrial transcription factor A (TFAM) is required for expression and maintenance of mtDNA. Tfam heterozygous knock-out (Tfam(+/-)) mice show mild mtDNA depletion, but have no overt phenotypes. We show that Tfam(+/-) mouse cells and tissues not only possess less mtDNA but also increased oxidative mtDNA damage. Crossing Tfam(+/-) mice to the adenomatous polyposis coli multiple intestinal neoplasia (APC(Min/+)) mouse cancer model revealed that mtDNA instability increases tumor number and growth in the small intestine. This was not a result of enhancement of Wnt/ß-catenin signaling, but rather appears to involve a propensity for increased mitochondrial ROS production. Direct involvement of mitochondrial ROS in intestinal tumorigenesis was shown by crossing APC(Min/+) mice to those that have catalase targeted to mitochondria, which resulted in a significant reduction in tumorigenesis in the colon. Thus, mitochondrial genome instability and ROS enhance intestinal tumorigenesis and Tfam(+/-) mice are a relevant model to address the role of mtDNA instability in disease states in which mitochondrial dysfunction is implicated, such as cancer, neurodegeneration, and aging.

Preferential selection of the 5'-terminal start codon on leaderless mRNAs by mammalian mitochondrial ribosomes.

Mammalian mitochondrial mRNAs are basically leaderless, having few or no untranslated nucleotides prior to the 5'-start codon. We demonstrate here that mammalian mitochondrial 55 S ribosomes preferentially form initiation complexes at a 5'-terminal AUG codon over an internal AUG. The preferential use of the 5'-start codon is also seen on mitochondrial 28 S small subunits, which suggests that mitochondrial translation initiation on leaderless mRNAs does not require the large ribosomal subunit. The selection of the 5'-AUG is dependent on the presence of fMet-tRNA and is enhanced by the presence of the mitochondrial initiation factor IF2(mt). In prokaryotes, IF3 is believed to antagonize initiation on leaderless mRNAs. However, IF3(mt) stimulates initiation complex formation on leaderless mRNAs when tested with 55 S ribosomes. The addition of even a few nucleotides 5' to the AUG codon significantly reduces the efficiency of initiation, highlighting the importance of the leaderless or nearly leaderless nature of mitochondrial mRNAs. In addition, very few initiation complexes could form on a hybrid mRNA construct consisting of tRNA(Met) attached at the 5'-end of a mitochondrial protein-coding sequence. This observation demonstrates that post-transcriptional processing must occur prior to translation in mammalian mitochondria.

Analysis of the functional consequences of lethal mutations in mitochondrial translational elongation factors.

Mammalian mitochondria synthesize a set of thirteen proteins that are essential for energy generation via oxidative phosphorylation. The genes for all of the factors required for synthesis of the mitochondrially encoded proteins are located in the nuclear genome. A number of disease-causing mutations have been identified in these genes. In this manuscript, we have elucidated the mechanisms of translational failure for two disease states characterized by lethal mutations in mitochondrial elongation factor Ts (EF-Ts(mt)) and elongation factor Tu (EF-Tu(mt)). EF-Tu(mt) delivers the aminoacyl-tRNA (aa-tRNA) to the ribosome during the elongation phase of protein synthesis. EF-Ts(mt) regenerates EF-Tu(mt):GTP from EF-Tu(mt):GDP. A mutation of EF-Ts(mt) (R325W) leads to a two-fold reduction in its ability to stimulate the activity of EF-Tu(mt) in poly(U)-directed polypeptide chain elongation. This loss of activity is caused by a significant reduction in the ability of EF-Ts(mt) R325W to bind EF-Tu(mt), leading to a defect in nucleotide exchange. A mutation of Arg336 to Gln in EF-Tu(mt) causes infantile encephalopathy caused by defects in mitochondrial translation. EF-Tu(mt) R336Q is as active as the wild-type protein in polymerization using Escherichia coli 70S ribosomes and E. coli [(14)C]Phe-tRNA but is inactive in polymerization with mitochondrial [(14)C]Phe-tRNA and mitochondrial 55S ribosomes. The R336Q mutation causes a two-fold decrease in ternary complex formation with E. coli aa-tRNA but completely inactivates EF-Tu(mt) for binding to mitochondrial aa-tRNA. Clearly the R336Q mutation in EF-Tu(mt) has a far more drastic effect on its interaction with mitochondrial aa-tRNAs than bacterial aa-tRNAs.

The effect of spermine on the initiation of mitochondrial protein synthesis.

Polyamines are important in both prokaryotic and eukaryotic translational systems. Spermine is a quaternary aliphatic amine that is cationic under physiological conditions. In this paper, we demonstrate that spermine stimulates fMet-tRNA binding to mammalian mitochondrial 55S ribosomes. The stimulatory effect of spermine is independent of the identity of the mRNA. The degree of stimulation of spermine is the same at all concentrations of mitochondrial initiation factor 2 (IF2(mt)) and mitochondrial initiation factor 3 (IF3(mt)). This observation indicates that IF2(mt) and IF3(mt), while essential for initiation, are not the primary components of the translation initiation system affected by spermine. IF3(mt) dissociates 55S ribosomes detectably in the absence of spermine, but this effect is strongly inhibited in the presence of spermine. This observation indicates that the positive effect of spermine on initiation is not due to an increase in the availability of the small subunits for initiation. Spermine also promotes fMet-tRNA binding to small subunits of the mitochondrial ribosome in the presence of IF2(mt). The major effect of spermine in promoting initiation complex formation thus appears to be on the interaction of fMet-tRNA with the ribosome.

Toxicity and Immunogenicity of a Tardigrade Cytosolic Abundant Heat Soluble Protein in Mice.

Tardigrades are microscopic animals well-known for their stress tolerance, including the ability to survive desiccation. This survival requires cytosolic abundant heat soluble (CAHS) proteins. CAHS D protects enzymes from desiccation- and lyophilization-induced inactivation in vitro and has the potential to stabilize protein-based therapeutics, including vaccines. Here, we investigate whether purified recombinant CAHS D causes hemolysis or a toxic or immunogenic response following intraperitoneal injection in mice. CAHS D did not cause hemolysis, and all mice survived the 28-day monitoring period. The mice gained weight normally and developed anti-CAHS D antibodies but did not show upregulation of the inflammatory cytokines interleukin-6 and tumor necrosis factor alpha. In summary, CAHS D is not toxic and does not promote an inflammatory immune response in mice under the conditions used here, suggesting the reasonability of further study for use as stabilizers of protein-based therapeutics.

Evidence for an active role of IF3mt in the initiation of translation in mammalian mitochondria.

Mitochondrial translational initiation factor 3 (IF3(mt)) is a 29 kDa protein that has N- and C-terminal domains, homologous to prokaryotic IF3, connected by a linker region. The homology domains are preceded and followed by short extensions. No information is currently available on the specific residues in IF3(mt) important for its activity. On the basis of homology models of IF3(mt), mutations were designed in the N-terminal, C-terminal, and linker domains to identify the functionally important regions. Mutation of residues 170-171, and 175 in the C-terminal domain to alanine resulted in a nearly complete loss of activity in initiation complex formation and in the dissociation of mitochondrial 55S ribosomes. However, these mutated proteins bind to the small (28S) subunit of the mammalian mitochondrial ribosome with K(d) values similar to that of the wild-type factor. These mutations appear to lead to a factor defective in the ability to displace the large (39S) subunit of the ribosome from the 55S monosomes in an active process. Other mutations in the N-terminal domain, the linker region, and the C-terminal domain had little or no effect on the ability of IF3(mt) to promote initiation complex formation on mitochondrial 55S ribosomes. Mutation of residues 247 and 248 in the C-terminal extension abolished the ability of IF3(mt) to reduce the level of binding of fMet-tRNA to the ribosome in the absence of mRNA. Our results suggest that IF3(mt) plays an active role in initiation of translation.

Mitochondrial redox sensing by the kinase ATM maintains cellular antioxidant capacity.

Mitochondria are integral to cellular energy metabolism and ATP production and are involved in regulating many cellular processes. Mitochondria produce reactive oxygen species (ROS), which not only can damage cellular components but also participate in signal transduction. The kinase ATM, which is mutated in the neurodegenerative, autosomal recessive disease ataxia-telangiectasia (A-T), is a key player in the nuclear DNA damage response. However, ATM also performs a redox-sensing function mediated through formation of ROS-dependent disulfide-linked dimers. We found that mitochondria-derived hydrogen peroxide promoted ATM dimerization. In HeLa cells, ATM dimers were localized to the nucleus and inhibited by the redox regulatory protein thioredoxin 1 (TRX1), suggesting the existence of a ROS-mediated, stress-signaling relay from mitochondria to the nucleus. ATM dimer formation did not affect its association with chromatin in the absence or presence of nuclear DNA damage, consistent with the separation of its redox and DNA damage signaling functions. Comparative analysis of U2OS cells expressing either wild-type ATM or the redox sensing-deficient C2991L mutant revealed that one function of ATM redox sensing is to promote glucose flux through the pentose phosphate pathway (PPP) by increasing the abundance and activity of glucose-6-phosphate dehydrogenase (G6PD), thereby increasing cellular antioxidant capacity. The PPP produces the coenzyme NADPH needed for a robust antioxidant response, including the regeneration of TRX1, indicating the existence of a regulatory feedback loop involving ATM and TRX1. We propose that loss of the mitochondrial ROS-sensing function of ATM may cause cellular ROS accumulation and oxidative stress in A-T.

Mitohormesis in Mice via Sustained Basal Activation of Mitochondrial and Antioxidant Signaling.

Transient mitochondrial stress can promote beneficial physiological responses and longevity, termed "mitohormesis." To interrogate mitohormetic pathways in mammals, we generated mice in which mitochondrial superoxide dismutase 2 (SOD2) can be knocked down in an inducible and reversible manner (iSOD2-KD mice). Depleting SOD2 only during embryonic development did not cause post-natal lethality, allowing us to probe adaptive responses to mitochondrial oxidant stress in adult mice. Liver from adapted mice had increased mitochondrial biogenesis and antioxidant gene expression and fewer reactive oxygen species. Gene expression analysis implicated non-canonical activation of the Nrf2 antioxidant and PPARγ/PGC-1α mitochondrial signaling pathways in this response. Transient SOD2 knockdown in embryonic fibroblasts from iSOD2-KD mice also resulted in adaptive mitochondrial changes, enhanced antioxidant capacity, and resistance to a subsequent oxidant challenge. We propose that mitohormesis in response to mitochondrial oxidative stress in mice involves sustained activation of mitochondrial and antioxidant signaling pathways to establish a heightened basal antioxidant state.

Overexpression and purification of mammalian mitochondrial translational initiation factor 2 and initiation factor 3.

Two mammalian mitochondrial initiation factors have been identified. Initiation factor 2 (IF2(mt)) selects the initiator tRNA (fMet-tRNA) and promotes its binding to the ribosome. Initiation factor 3 (IF3(mt)) promotes the dissociation of the 55S mitochondrial ribosome into subunits and may play additional, less-well-understood, roles in initiation complex formation. Native bovine IF2(mt) was purified from liver a number of years ago. The yield of this factor is very low making biochemical studies difficult. The cDNA for bovine IF2(mt) was expressed in Escherichia coli under the control of the T7 polymerase promoter in a vector that provides a His(6)-tag at the C-terminus of the expressed protein. This factor was expressed in E. coli and purified by chromatography on Ni-NTA resins. The expressed protein has a number of degradation products in partially purified preparations and this factor is then further purified by high-performance liquid chromatography or gravity chromatography on anion exchange resins. IF3(mt) has never been purified from any mammalian system. However, the cDNA for this protein can be identified in the expressed sequence tag (EST) libraries. The portion of the sequence encoding the region of human IF3(mt) predicted to be present in the mitochondrially imported form of this factor was cloned and expressed in E. coli using a vector that provides a C-terminal His(6)-tag. The tagged factor is partially purified on Ni-NTA resins. However, a major proteolytic fragment arising from a defined cleavage of this protein is present in these preparations. This contaminant can be removed by a single step of high-performance liquid chromatography on a cation exchange resin. Alternatively, the mature form of IF3(mt) can be purified by two sequential passes through a gravity S-Sepharose column.

Aging: it's SIRTainly possible to restore mitochondrial dysfunction.

Mitochondrial dysfunction is strongly associated with aging. A recent study shows that reduced nuclear SIRT1 activity initiates age-related mitochondrial decline through a signaling pathway that perturbs expression of genes encoded by mitochondrial DNA. This reversible pathway has potential anti-aging therapeutic value.

In situ kinetic trapping reveals a fingerprint of reversible protein thiol oxidation in the mitochondrial matrix.

Reactive oxygen species (ROS) are released at the mitochondrial inner membrane by the electron transport chain (ETC). Increasing evidence suggests that mitochondrial H2O2 acts as a signaling molecule and participates in the (feedback) regulation of mitochondrial activity and turnover. It seems likely that key mitochondrial components contain redox-sensitive thiols that help to adapt protein function to changes in electron flow. However, the identity of most redox-regulated mitochondrial proteins remains to be defined. Thioredoxin 2 (Trx2) is the major protein-thiol-reducing oxidoreductase in the mitochondrial matrix. We used in situ mechanism-based kinetic trapping to identify disulfide-exchange interactions of Trx2 within functional mitochondria of intact cells. Mass spectrometry successfully identified known and suspected Trx2 target proteins and, in addition, revealed a set of new candidate target proteins. Our results suggest that the mitochondrial protein biosynthesis machinery is a major target of ETC-derived ROS. In particular, we identified mitochondrial methionyl-tRNA synthetase (mtMetRS) as one of the most prominent Trx2 target proteins. We show that an increase in ETC-derived oxidants leads to an increase in mtMetRS oxidation in intact cells. In conclusion, we find that in situ kinetic trapping provides starting points for future functional studies of intramitochondrial redox regulation.